1. Field of the Invention
The present invention relates to an image forming apparatus using a dielectric member and, more particularly, to a technique of removing a charge from the dielectric member, and to an image forming apparatus to which such a technique is applied.
2. Description of the Related Art
A known image forming apparatus employing a dielectric member and a charge-removing technique, provides electrical conductivity between the dielectric member and a recording electrode. A signal voltage is applied to the recording electrode to form a toner image on the dielectric member. Such an apparatus is disclosed in, for example, Japanese Examined Patent Publication No. 51-46707 and corresponding U.S. Pat. No. 3,914,771. As shown in FIG. 12, a magnetic, electrically-conductive toner 1 is transferred by a rotary magnet 2 onto a non-magnetic cylinder 3, made of, for example, stainless steel, and passed onto a recording electrode 4 made of a magnetic material. A voltage is applied between the recording electrode 4 and an electrically-conductive layer 7 of a recording medium M. The recording medium M has an insulating layer 6 formed on the surface thereof, which surface constitutes the dielectric member. A charge is thus induced to the toner which electrostatically adheres to the recording medium M, whereby an image is formed.
There has been proposed an image forming device to which the recording technique mentioned above is applied, and in which the above mentioned recording electrode is utilized. FIG. 11 shows the entire structure of such a display device. In this Figure, numeral 1 denotes electrically-conductive toner; 4, a recording electrode; M, a recording medium; 8, a brush-like cleaning member; 9, a magnet; 10, a toner container; 11, a member for supporting the recording medium; 12, a main frame; and 13, a record control section. The electrically-conductive toner 1 adheres or does not adhere to the recording medium M in accordance with a signal voltage applied from the recording electrode 4. Thereby, a toner image is formed on the recording medium M. For example, when a signal voltage of +40 V is applied from the record control section 13, the toner adheres to the recording medium M, whereas when no voltage is applied, the toner does not adhere, thus forming the toner image on the recording medium M.
The toner image which has been formed on the recording medium M is displayed, and then scraped by the brush-like cleaning member 8, and thus removed from the recording medium M. Conventionally, during the above-described operation a predetermined voltage ranging from -3 to -5 V is applied to a brush 8' of the cleaning member 8. Furthermore, there has been proposed a humidity sensor which measures the amount of environmental moisture. A voltage is applied to the cleaning member 8 according to the amount of environmental moisture. These inventions are disclosed in U.S. Pat. Nos. 4,788,564; 4,887,103; 4,989,021; 4,910,538; 5,077,566; and patent application Ser. No. 545,645 (filed on Jun. 29, 1990) now U.S. Pat. No. 5,089,832 issued Feb. 18, 1992.
In the above-noted known arrangements, as the cleaning member scrapes and collects the residual toner, it generates friction while sliding on the recording medium. In addition, a charge-removing member in contact with an electrically-conductive member of the recording medium removes the charge of the dielectric member while the charge-removing member is in contact with an electrically-conductive material, such as a brush or a rubber roller. One disadvantage to the known arrangements is that when the cleaning member slides on the recording medium and friction is produced, a charge is generated on the recording medium because of a frictional static build-up. As a result, when the recording medium passes a recording portion, the toner adheres to the charge generated during cleaning, thereby contaminating the recording medium.
Another disadvantage to the known arrangements is that a charge, which is opposite to the charge generated by friction while the cleaning member slides on the recording medium, is induced and generated inside the recording medium. If the area where the charge-removing member is in contact with the electrically-conductive member is small, the induced charge and the charge generated inside the recording medium during recording cannot be removed completely. Therefore, a previously-recorded image is partially developed and produced on the recording medium, i.e., a so-called ghost image is produced.
To improve the efficiency with which a charge is removed, it is possible to employ both a method for increasing the area where the charge-removing member is in contact with the electrically-conductive member of the recording medium, and a method for forcibly pressing the charge removing member against the electrically-conductive member of the recording medium. In these methods, however, while the recording medium is being driven, its load is increased because of the friction and pressing force. As a result, the capacity of the driving motor for transferring the recording medium must be increased, thus increasing operational noise and power consumption.
There are two problems with the method in which the humidity sensor measures moisture and a voltage is accordingly applied to the cleaning member. First, the humidity sensor may respond inaccurately. Second, the amount of voltage to be applied to the cleaning member is uniformly determined regardless of the electrical resistance of the surface of the recording medium. Thus an inappropriate amount of voltage may be applied.